starting material and the oxidation was carried out in a basic solution, the polarogram showed no wave for selenium(1V) in the calcium selenate. Reduction of Selenium(1V) by Sulfur Dioxide. It was found that the reduction of selenium(1V) can be accomplished quantitatively in 6N hydrochloric acid, 3N sulfuric acid, and 3N perchloric acid using sulfur dioxide which was generated internally from sodium sulfite or added as a gas. The results as summarized in Table I indicate that the most consistent results are obtained when perchloric acid is used. The average deviation of the mean of the averages is about 1 ppt which is well within the generally accepted limits for good precision. The slightly higher values obtained when hydrochloric and sulfuric acids were used maj’ have been the result of occlusion because the precipitates in perchloric acid solutions were more finely divided.
The polarograms from the filtrates from each of the determinations showed no selenium(1V) wave, which indicated that no selenium was present in amounts larger than 79 ppm. The results from the atomic absorption determination are recorded in Table I1 and also indicate a quantitative reduction of selenium(1V) by sulfur dioxide. Determination of Selenium(1V) in the Presence of Selenium (VI). When perchloric acid was used as the acid medium, an average of 41.05 0.01% of selenium was obtained as compared to an average of 41.04 f 0.03% when no calcium selenate was present. The polarograms showed no selenium(1V) wave, indicating a quantitative separation of selenium(1V) in the presence of selenium(V1).
*
RECEIVED for review December 23, 1968. Accepted March 20, 1969.
Spectrophotometric Determination of Hafnium as Reduced Molybdosulfatohafnic Acid C. C. Clowers, Jr., and J. C. Guyon Department of Chemistry, University of Missouri, Columbia, Mo. 65201 SEVERAL chromogenic reagents have been utilized in spectrophotometric determinations of hafnium (1-10). The zirconium work of Grosscup ( I ] ) , Liberti (12), and Shakova (13), and studies by Dehne (14) indicated the possibility of formation of a sulfato heteropoly and species involving hafnium. The purpose of this work was two-fold: to furnish qualitative existence of a simple heteropoly molybdohafnic acid or a related species, and to develop a quantitative spectrophotometric method for the determination of hafnium based on some mechanism of formation of its heteropoly molybdate. EXPERIMENTAL
Apparatus. All spectrophotometric measurements were made using a Cary Model 12 automatic recording spectrophotometer with 1.000 + 0.002-cm fused quartz cells. All pH measurements were made using either a Beckman Zeromatic or Beckman Expanded-Scale direct-reading pH meter. (1) Y . Hoshino, Nippoii Kagakir Zasshi, 80, 738 (1959). (2) K. N. Bagdasarov, Ref. Zh, Khim., 1963, Abstract No. 7G82. (3) L. I. Kononenko and N. S. Poluektov, Zacodsk. Lab., 28, 794 ( 1962). (4) A. D. Horton, ANAL.CHEM., 25,1331 (1953). ( 5 ) K. L. Cheng, Talanta, 2, 81 (1959). (6) G. Banerjee, Anal. Chim. Acta, 16, 62 (1957). (7) F. R. Sheyanova and V. L. Ganina, Trudy p o Khimii i Khim. Tekhnol., 3, 101 (1960). (8) K. L. Cheng, Anal. Chim. Acta, 28,41 (1963). (9) S. D. Biswas, Talarita, 12, 119 (1965). (10) K. Pan, A. Lin, S. Lin, Y . Wu, and E. Chen, J. Chinese Chem. SOC.,10, 24 (1963). (11) C. G. Grosscup, J. Amer. Chem. SOC.,52, 5154 (1930). (12) A. Liberti, La Ricerca Sei., 25, 880 (1955). (13) Z. F. Shakova, Zh. Neorg. Khim., 6, 330 (1961). (14) G. C. Dehne with M. G. Mellon, ANAL.CHEM.,35, 1382 (1963).
1140
ANALYTICAL CHEMISTRY
Temperature was controlled with a water bath in conjunction with a Sargent Model 3554 thermoregulating unit. Reagents. Stock solutions of ammonium heptamolybdate, approximately 5 % (w/v), Na2S04, approximately 10% (w/v), and chlorostannous acid, 0.025M, were used. A stock hafnium(1V) solution was prepared from HfO(NO& and standardized by precipitation of the hydrous oxide with ignition to HfOn. Recommended Procedure. PREPARATION OF CALIBRATION CURVE. Transfer 0.0, 0.5, 1.0, 2.0, 3.0, and 5.0 ml of a standard solution containing approximately 1.O mg/ml of hafnium to 150-ml beakers. Add 4.0 ml of a 10% (w/v) solution of NazS04,adjust the volume to approximately 35 ml with water and add 10 ml of a 5 (w/v) solution of ammonium heptamolybdate. Adjust the pH in each beaker to 1.3, transfer the solutions to 100-ml volumetric flasks and place the flasks in a water bath (45 “C f 0.5) for 90 minutes. Add, by means of hypodermic syringes, 5.0 ml of 1:1 sulfuric acid and, exactly 10 seconds later, 3.0 ml of 0.025Mchlorostannous acid to the solutions while still warm. Dilute to the mark, mix, and read the absorbance at 725 mp exactly 20 minutes after addition of reductant, using distilled water as the reference. Construct a calibration curve of absorbance cs. concentration. GENERAL PROCEDURE.Dissolve the sample to be analyzed for hafnium and treat the resulting solution to remove interfering ions. Concentrate the resulting solution to about 30 ml. Adjust the amount of sulfate present to between 225 and 475 mg by adding sodium sulfate solution. Add 10 ml of 5 (w/v) ammonium heptamolybdate solution and proceed as outlined under “preparation of calibration curve,” beginning with the pH adjustment step. Effect of Experimental Variables. Preliminary studies indicated that reduction of the heteropoly species would be necessary if the method were to have sufficient sensitivity. The absorption spectrum of the reduced species showed a maximum at 725 mp, and all measurements were made at this wavelength.
a70t
0.61
10
ZD
30
40
50
00
70
80
90
Time(min.I
Figure 1. Effect of time of complex formation ml of 5%w/v
8.0 10.0 1ZO Added Ammonium Hepfamolybdale
140
Figure 2. Effect of molybdate concentration TEMPERATURE. Sample solutions containing hafnium and blanks containing no hafnium were treated identically. To 150-ml beakers were added 2 ml of the hafnium stock solution, 5 ml of 10% sodium sulfate solutions, 25 ml of water, and 10 ml of a 4 % solution of ammonium heptamolybdate. The pH was adjusted to 1.3 with 1:l HzS04 and the solutions were transferred to 100-ml volumetric flasks and placed in a water bath at the different temperatures indicated on the right side of Figure 1. The flasks were held there for a length of time indicated by the abscissa. At this point, 4.0 ml of 1:1 &So4 was added via hypodermic syringe, followed by 3.0 ml of the tin(I1) reductant exactly 15 seconds later. The absorbances were read after a lapse of 5 minutes. Interest in sensitivity and precision requires elevation of the bath temperature to 45 “C for at least 1 hour; subsequent measurements were made at this temperature after a period of 90 minutes. PH. The optimum pH was selected as that which showed a maximum in absorbance difference between a series of samples containing hafnium and a series of “blanks” containing no hafnium. These data showed that the reducible intermediate is best formed in the rather narrow pH range of 1.25 to 1.40. For further work, all solutions were adjusted as closely as possible to pH 1.30 before the temperature was elevated. MOLYBDATE CONCENTRATION. It was found that 10 ml of a 5 % ammonium heptamolybdate solution was optimum for future measurements (Figure 2). This represents an approximately 270-fold excess of molybdenum over hafnium. Inasmuch as reactions involving formation of heteropoly complexes rarely go to completion, entire complexation of hafnium was expected to require a large excess of molybdate. STABILITY OF UNREDUCED COMPLEX.It was necessary to ascertain how stable the unreduced heteropoly complex was in the presence of strong sulfuric acid. For this study the absorbance differences were observed as a function of the time lapse between addition of strong sulfuric acid and reductant. Other variables were held constant. Ten seconds was chosen as the optimum time interval between addition of reagents in the reduction step. STABILITY OF REDUCED COMPLEX.The absorbance of a reduced solution containing hafnium was studied as a function of time after reduction at 725 mp. The final blue hue was shown to be fairly stable up to 1 hour, with a slight maximum sensitivity between 5 and 30 minutes. An intermediate time of 20 minutes was chosen for making subsequent final absorbance readings. SULFATE CONCENTRATION. It was found that insufficient sulfate was added to the system through preparation of the standard hafnium solution and by pH adjustment. An
additional 4.0 ml of 10% sodium sulfate solution was chosen for subsequent use. SULFURICACID CONCENTRATION. If reduction of the solution with chlorostannous acid is made immediately after the complex is allowed to form under the proper conditions, most of the absorbance at 725 mp will be due to the excess reduced isopoly molybdate present. Addition of sufficient H2S04 will largely prevent reduction of this excess to a blue, but will also destroy the reducible heteropoly complex containing hafnium. The rate of destruction of the latter, however, is much slower than the rate of conversion of the excess isopoly species to nonreducible substances. A volume of 5.0 ml of 1:1 HsS04 was selected for use in future investigations.
Table I. Effect of Diverse Ions Ion
Added as
Concn added, ppm
Concn permitted,
100 100
100 100
c1co2+ Cr 3+ cu2+ CNFez+ Fe 3+ FI-
100 100 100 100
100 100 100 100 100 100
Zn 2+ Zr (+
100 100 100 0 100 100 75 25 0
0 0 0
100 100
100 100
100 0
100 100 100 100 100
75 100
100
We+
100
0 50 0 0 25 50
100 100
Ni2+ NO3P5+ SCNTh4+
ppm
VOL. 41, NO. 8,JULY 1969
0
100 0
1141
NATUREAND CONCENTRATION OF REDUCTANT. Several mild reducing agents were substituted for chlorostannous acid in the established procedure. (One-amino-2-naphthol4-sulfonic acid-sodium sulfite) and L-ascorbic acid gave pale blue reduction products, and then only very slowly. Iron(I1) acted more rapidly, but the color was only slightly more intense. Hypophosphorus acid behaved similarly to l-amino2-naphthol-4-sulfonic acid and ascorbic acid. Hydroxylamine hydrochloride failed to reduce a hafnium-containing solution at all. Hydrazine hydrochloride and chlorostannous acid gave the best results. However, the blue color in the hydrazine solution became dark green after 8 to 10 minutes, and the corresponding blank changed color from pale blue to orange-yellow in approximately the same length of time. On this basis, chlorostannous acid was chosen as the most satisfactory reductant, and 3.0 ml of a 0.025M solution was selected for final use. ADHERENCE TO BEER’SLAW. The system obeys Beer’s law over the approximate range of 8 to 50 ppm of hafnium. The deviation at concentrations below the lower limit is probably caused by incomplete conversion of metal ion to the absorbing species. The upper limit approached the absorbance limit of our instrument. The molar absorptivity of the complex was calculated to be 6.7 X l o 3 l/mole-cm. EFFECTOF DIVERSE IONS. The results of this study are shown in Table I. Ions investigated were considered tolerable at the stated concentration level if the absorbance of a standard hafnium-containing solution was affected by no more than =t27$ Insoluble precipitates were formed with Ag+, Pb2+, Ba2+,Bi3+, Snzf, and Sb3+under the conditions used. Either by preventing reduction or by forming reducible complexes themselves, HSO4-, Poda-, VOa-, citrate, NOZ-, and tartrate interfere seriously.
DISCUSSION AND CONCLUSIONS Evidence has been presented for interaction between hafnium(1V) and isopoly molybdate in our system. The resulting species is probably of the normal 12:l type-for example HfO(Mo301&~-, analogous to the normal 12-molybdozirconic acid system. It was also shown, however, that this resulting species was not reducible to a blue hue unless sulfate was present. Sulfate complexes of heteropoly anions have been reported (15, 16), and it is presumed that a sulfatomolybdate is a necessary intermediate. This is concluded from the fact that an increase in temperature was a necessary condition for reducible complex formation, but not a sufficient one. The nonreducible sulfatomolybdate must then react with the hafnium(1V) at a lower temperature to give the reducible ternary species. Actual reduction of the intermediate to measure hafnium(1V) concentration must be preceded by elimination of excess isopoly molybdate which is also reducible to a blue hue. Sulfuric acid, when present in concentrations greater than 2N accomplished this. It was concluded that in this analytical system, the reducible intermediate containing hafnium must constitute a “mixed” heteropoly acid, the exact nature of which reduces to a problem of speculation on structure in the aqueous acidic solution. RECEIVED for review January 22, 1969. Accepted April 1, 1969. (15) G. C . Dehne, Ph.D. Thesis, Purdue University, Lafayette,
Ind., 1963. (16) K. Schriever and R. Toussaint, Chem. Ber., 91, 2639 (1958).
I CORRESPONDENCE
I
Note on the Polarographic Theory for an ECE Mechanism SIR: Chemical reactions following charge transfer and producing a species which enters a second charge transfer reaction are well known in polarography as well as with other electrochemical methods. These chemical reactions may proceed as heterogeneous processes--e.g., proton transfer to anionic species generated by the first reduction process and being adsorbed at the interphase-as well as by a homogeneous mechanism. First or second order kinetics-e.g., dismutation-may apply in this latter case. If the rate of reaction is low, then the kinetics may be followed by controlled potential electrolysis giving rise to an accumulation of the intermediate product (or, if dismutation occurs, to a steady state concentration of the depolarizer) in the bulk of the solution. A direct electrochemical response, however, is only observed, if the electrochemical response time-e.g., the polarographic drop time-is not too small as compared with the time constant of the reaction. Polarographic currents will then exceed those due to the first charge transfer process. If the interposed chemical reaction constitutes an equilibrium, which is in favor of the electroinactive species B,
On the other hand, if kb < k,, the transformation of B to the electroactive C causes the polarographic limiting currents to be intermediate between those corresponding to the transfer of nl and (n, n2)electrons, respectively, but different from both limiting cases sufficiently for the evaluation of k,, only as far as -2 < log ( k f 7 )< +2, where 7 is the polarographic drop time. Taking kb = 0 and (for abbreviation) kf = k , the problem is described by
+
Da =
Db = -kb Dc =
1142
ANALYTICAL CHEMISTRY
(1)
+kb
where a , 6, c denote the concentrations of species A , B, C, and the operator 9 is given by
The initial and boundary conditions are t = 0,x 2 0 t 2 0,x’m
the rates of both forward and backward reactions being very fast, then the well-known treatment developed by Koutecky is applicable.
0
t>O,x=O:
a=aL;b=
a = c =
0;
aa -+ ax
c = O
ab
- =axo
(3)
